1. Field of the Invention
The present invention relates to measuring electric characteristics of a semiconductor wafer, such as a C-V curve.
2. Description of the Related Art
Manufacture of semiconductors, especially, of LSIs includes many process steps. Each process step must be meticulously executed to maintain the highest quality of the semiconductors. Manufacturers of semiconductors therefore carry out various tests to confirm that after each process step, the semiconductor maintains its electrical characteristic at a desired level.
C-V curve measurement is one method generally used for evaluating determine when the poor C-V characteristics are the result of a having MIS (Metal Insulator Semiconductor) structure.
FIGS. 1(a) through 1(c) are explanatory views showing a conventional method for obtaining a C-V curve. A semiconductor of MIS structure shown in FIG. 1(a) includes a semiconductor substrate 101, an oxide film 102 formed on the substrate, and an electrode 201 formed on the film 102. Another electrode 202 is also formed on the opposite surface of the substrate 101.
A total electrostatic capacity Ct between the two electrodes 201 and 202 is expressed by a series connection of a capacity Cs of the substrate 101 and a capacity Ci of the oxide film 102 as seen in FIG. 1(b).
A solid line and a broken line in FIG. 1(c) respectively represent an ideal or calculated C-V curve and an actual or measured C-V curve. In this C-v curve measurement, a flat band voltage Vfb is first obtained according to a shift between the ideal curve and the actual curve, and the quality of the semiconductor and manufacturing process thereof are then evaluated based on the flat band voltage Vfb.
The conventional C-V curve measurement thus requires the electrode 201 on the oxide film 102.
The step of forming the electrode 201 on the oxide film 102, however, sometimes causes contamination and damage in the oxide film 102 with impurity ions such as sodium. As generally known, the C-V characteristics depend on the concentration of the impurity ions in the oxide film 102. It has been difficult therefore to the process steps of manufacturing semiconductors poor quality of the oxide film 102 (resulting from a poor oxide film formation process (or the result of a poor quality of the electrode resulting from the electrode formation process.
The process of forming an oxide film includes a number of steps, such as cleaning of a semiconductor substrate, thermal oxidation, and stabilizing heat treatment of an oxide film. The adequacy of each step may be determined by evaluating the C-V curve after each step. However, evaluation of each step requires the formation of a test electrode on the substrate after each step; for example, C-V curve measurement after cleanup treatment of the substrate requires formation of a test electrode on the cleaned substrate.
According to the conventional method described above, a test electrode is required in each evaluation of electric characteristics of the semiconductor wafer. Formation of a test electrode on the semiconductor wafer is, however, both time and labor consuming, and moreover affects the electric characteristics of the semiconductor wafer.
Another method using a C-t curve, that is, variation of electrostatic capacity over time, is sometimes used for evaluating the characteristics at the surface portion of a semiconductor wafer. This method also requires a test electrode to be formed on the semiconductor as is the case with the C-V measurement; namely, it has the same problems as above.